A semiconductor device represented by an ASIC (Application Specific Integrated Circuit) normally has an internal circuit region and an input/output circuit region arranged around it. A circuit having a desired function such as signal processing is formed in the internal circuit region. This circuit is formed by arranging a plurality of cells using a plurality of semiconductor elements such as MOS transistors (Metal Oxide Semiconductor transistors) and connecting them by wiring.
The input/output circuit region transmits signals between the internal circuit region and a device outside the semiconductor device or supplies a power and reference potential (to be referred to as ground hereinafter) necessary for the ground in the internal circuit region and input/output circuit region to operate. The signals and power supply/ground in the input/output circuit region and the internal circuit region are connected by wiring.
Like the internal circuit region, the input/output circuit region is also formed by arranging a plurality of cells for input/output circuits, in which a plurality of semiconductor elements such as transistors are arranged. The semiconductor device and an external device are connected through pads or semiconductor packages in the input/output cells. Power supply/ground wiring to supply a power supply/ground to the semiconductor elements in the internal circuit region and input/output circuit region is present in the semiconductor device independently of the signal wiring. To stabilize the potential immediately under the channel of a MOS transistor, the power supply/ground wiring connects the semiconductor substrate to the power supply wiring in a p-type MOS (to be referred to as a PMOS hereinafter) or connects the semiconductor substrate to the ground wiring in an n-type MOS (to be referred to as an NMOS hereinafter).
In the above-described structure, a noise current which is generated in the power supply/ground wiring in the semiconductor device when it operates poses a problem. A noise current generated by the operation of a circuit flows through the power supply/ground wiring and is injected from the connection portion between the semiconductor substrate and the power supply/ground wiring to the semiconductor substrate. As a result, the potential immediately under the channel of the MOS transistor varies and causes an operation error.
A technique is conventionally known in which a guard ring absorbs noise to prevent any operation error caused by noise propagation in the semiconductor substrate or any operation error or breakdown by latch-up.
In Japanese Patent Laid-Open Nos. 2002-246553, 2001-102525, and 1-103859, a guard ring surrounds the internal circuit serving as a noise source to prevent power supply/ground noise from propagating to another circuit through the semiconductor substrate and causing an operation error.
In Japanese Patent No. 3403981 and Japanese Patent Laid-Open No. 2001-127249, a thyristor or diode structure is intentionally inserted in a guard ring structure to increase the electrostatic destruction preventing effect.
Another problem of noise is radiation noise which is generated when the noise current of the power supply/ground in the internal circuit region propagates to the input/output circuit region through the power supply/ground wiring, and the power supply/ground noise of the internal circuit is superimposed on a signal of an input/output circuit, which is to be output to the outside. Conventionally, placing focus on the fact that the internal circuit and the power supply/ground of the input/output circuits are connected by wiring, the internal circuit and the power supply/ground of the input/output circuits are disconnected to avoid noise propagation from the internal circuit to the input/output circuits.
In the above-described prior arts, noise propagation through some internal circuits, some internal circuit wiring, and the semiconductor substrate or noise propagation through the wiring from the internal circuit to the input/output circuits is prevented. However, there is no measure against noise propagation through the semiconductor substrate between the internal circuit and the input/output circuits at all. Hence, the prior arts cannot prevent an operation error caused by propagation of noise, represented by SSN (Simultaneous Switching Noise) of the input/output circuits, which is generated by the input/output circuit operation to the internal circuit through the semiconductor substrate, or an increase in radiation noise caused by noise propagation from the internal circuit to the input/output circuits through the semiconductor substrate.